The invention relates generally to wireless communication systems and, more particularly, to demodulating spread spectrum signals.
A number of conventional wireless communication techniques have been developed. One common technique is code division multiple access (CDMA) in which multiple communications are simultaneously conducted over a radio-frequency (RF) spectrum. Example wireless communication devices (xe2x80x9csubscriber unitsxe2x80x9d) that have incorporated CDMA technology include cellular radiotelephones, satellite radiotelephones, PCMCIA cards for portable computers, personal digital assistants (PDAs) equipped with wireless communication capabilities, and the like.
A CDMA system may be designed to support one or more CDMA standards such as (1) the xe2x80x9cTIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (the IS-95 standard), (2) the xe2x80x9cTIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Stationxe2x80x9d (the IS-98 standard), (3) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the WCDMA standard), (4) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Project 2xe2x80x9d (3GPP2) and embodied in a set of documents including xe2x80x9cTR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d the xe2x80x9cC.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d and the xe2x80x9cC.S0024 CDMA2000 High Rate Packet Data Air Interface Specificationxe2x80x9d (the CDMA2000 standard), and (5) some other standards. A system that implements the High Rate Packet Data specification of the CDMA2000 standard is referred to herein as a high data rate (HDR) system. The HDR system is documented in TIA/EIA-IS-856, xe2x80x9cCDMA2000 High Rate Packet Data Air Interface Specification.xe2x80x9d Proposed wireless systems also provide a combination of HDR and low data rate services (such as voice and fax services) using a single air interface.
A typical CDMA communication system includes a number of mobile subscriber units that encode voice and data in CDMA waveforms. The subscriber units communicate with base stations, also referred to as base transceiver subsystems (BTS), cell stations, cell sites, or simply cells. A base station demodulates incoming CDMA waveforms received from subscriber units within a limited geographic region, and transmits outgoing CDMA waveforms to the subscriber units. A base station controller (BSC) provides an interface between the base stations and the public switched telephone network (PSTN) for routing the signals to other remote base stations or to any conventional telephony system. In general, transmission from the base station to the subscriber unit is referred to as the Forward CDMA Channel, and is sometimes called a downlink. Transmission from the subscriber unit to the base station is referred to as the Reverse CDMA Channel, and is sometimes called an uplink.
At any given moment, a single subscriber unit or base station may receive multiple replicas of the transmit signal, each replica possibly having a different amplitude, phase and time delay. The replicas are often caused by reflections of the transmit signal off of obstacles in the environment, such as buildings, trees, cars and people. The various replicas of the transmit signal are referred to as xe2x80x9cpaths,xe2x80x9d with the general characteristic referred to as xe2x80x9cmultipath.xe2x80x9d
The transmit signal of a CDMA communication system consists of a train of pulses called xe2x80x9cchips.xe2x80x9d More specifically, a transmitter generates a spread spectrum transmit signal by modulating an outbound serial stream of data with a pseudonoise (PN) code. The application of the PN code to the data produces a stream of chips. The resulting chips are transmitted according to some modulation scheme, such as quadrature phase shift keying (QPSK) modulation. In order to separate signals from multiple users, the receivers isolate the signal of the desired user by matching the signal to the corresponding PN code.
The rate that the PN code is applied is referred to as the chip rate, which is typically many times faster than the data rate. The duration of one pulse of the PN code is often used as a unit of measure referred to as chip time. The time delay between multiple received paths is often represented in chip time. A time delay of 1.5 chips between two paths, for example, indicates a time delay equal to 1.5 times the chip time of the transmit signal.
In order to reduce effects of signal fading and for other advantages, conventional CDMA receivers discriminate between multiple paths during demodulation. In particular, a typical CDMA receiver, commonly referred to as a RAKE receiver, contains a number of demodulation elements, and assigns the demodulation elements to track different received paths when the paths have a time separation greater than a threshold, such as approximately 1.5 chips or more. For shorter time separations, often referred to as xe2x80x9cunresolvable multipath spacing,xe2x80x9d conventional receivers typically do not assign multiple demodulation elements because the multiple paths often appear as a single mass of energy without readily detectable peaks. Similarly, if multiple paths shift from greater time separations to unresolvable multipath, the demodulation elements often converge to tracking a common time offset. As a result, conventional receivers typically deassign one of the demodulation elements.
In general, the invention is directed to assignment of demodulation elements within a spread spectrum system. When assigning demodulation elements to various paths, a receiver assembles a list of potential paths, and may include one or more additional xe2x80x9cvirtualxe2x80x9d paths to the list. A xe2x80x9cvirtualxe2x80x9d path, as used herein, refers to a path that is added to the list of candidate paths for assignment to a demodulation element even though a corresponding peak was not necessarily detected within the received spread spectrum signal. The receiver may selectively add one or more virtual paths having a time offset substantially near an assigned demodulation element, such as two or less chips away from the demodulation element in time. Furthermore, the receiver may define prioritizations for the demodulation element and the virtual paths by biasing their respective indicated signal strengths in the path list. By setting the signal strengths as such, the receiver prioritizes the assignment of demodulation elements around the multipath.
By making use of virtual paths, the receiver ensures that any free demodulation element may be used to demodulate the spread spectrum signal along a virtual path. Consequently, by considering virtual paths for tracking and demodulation, the receiver may more efficiently assign demodulation elements to improve performance in unresolvable multipath environments. The demodulation elements assigned to virtual paths can instantly demodulate the various paths in the event an unresolvable multipath situation exists. The demodulation elements can also instantly track multiple paths if the paths diverge to have greater time separations.
In one embodiment, the invention is directed to an apparatus, such as a wireless communication device. The apparatus comprises a plurality of demodulation elements that demodulate a spread spectrum signal. A search module measures a signal strength of the signal, and generates data indicative of a set of local maximums and corresponding time offsets. A controller coupled to the search module generates a list of signal paths based on the received data. The list of signal paths includes a first path having a time offset approximately equal to a time offset for one of the demodulation elements, and a second path having a short time separation from the first path. The controller includes the second path when at least one local maximum is detected within a threshold amount of time from the time offset of the demodulation element. The time separation between the first and second path may be, for example, less than 2 chips.
In another embodiment, the invention is directed to a method for assigning demodulation elements of a receiver within a spread spectrum system. A list of signal paths is generated from a received spread spectrum signal. A first path is added to the list of signal paths based on a current time offset for a demodulation element within the receiver. A second path is added to the list of signal paths, and has a time offset set to a predetermined amount of time from the time offset of the demodulation element.
In another embodiment, the invention is directed to a method for configuring demodulation elements that demodulate a spread spectrum signal at a chip rate. The method involves setting a time offset of a first demodulation element at approximately a predetermined amount of time from a time offset of a second demodulation element such that the time separation of the two demodulation elements is approximately two chips or less.
In another embodiment, the invention is directed to a method in which a list of signal paths is generated from a received spread spectrum signal. The method involves grouping a plurality of demodulation elements into at least one set of demodulation elements based on a corresponding time offset for each demodulation element. The list of paths is adjusted based on the number of demodulation elements in the set.
The invention is capable of providing a number of advantages. By making use of virtual paths when assigning demodulation elements, a subscriber unit or other component in a wireless communication device can achieve significant improvements in signal to noise ratio, power control and capacity in short multipath environments, such as a time separation of less than 1.5-2.0 chips. Improvements in signal to noise ratio have been observed, for example, of at least 1.75 dB in 1xc3x97 forward power controlled CDMA systems in non-handoff scenarios. Similarly, capacity improvements have been observed ranging up to 50% or more in multipath scenarios. The techniques may be advantageous in multipath environments in which multiple paths exist but have a small time separation such that the signal peaks are not readily detectable. Furthermore, the techniques may be particularly advantageous in xe2x80x9cshortxe2x80x9d multipath environments in which the time separation is between, for example, 0.5 chips and 2.0 chips.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.